What is RF transformer:
The most widely and most common core used for RF Transformer is toroid.
A toroid is circular doughnut shaped core made of special type of powdered iron material.
To make RF transformer, copper wire is wound on the toroid to create both primary and secondary windings.
The figure-1 depicts different transformer symbols as per core used.
Refer transformer basics and types>> and
RF transformer calculator>> for more information.

Following table lists popular manufacturers or vendors of RF transformer.